Films comprising metallocene catalyzed polyethylene

ABSTRACT

A high clarity film containing three layers wherein the polymer of each outer layer is the same or different polyethylene selected from polyethylenes having a density of at least 0.925 g/cc, and a molecular weight distribution of less than 4, and being further characterized by being substantially free of branches having 6 or more carbon atoms, polyethylenes of the outer layers optionally containing a fluoroelastomer.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 08/515,498 filed Aug. 15, 1995, now abandoned the disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to film of polymers produced from a monomerconsisting essentially of ethylene. In another aspect, the presentinvention relates to polyethylene film having a good balance ofphysical, processing, and optical properties.

BACKGROUND OF THE INVENTION

In its broadest sense, the term “film” as used herein refers toself-supporting materials having a wide range of thicknesses. Exampleswould include thicknesses in the range of 0.05 to about 40 mils, moretypically about 0.25 to about 5 mils (1 mil equals {fraction (1/1000)}of an inch). Films can be made using a variety of techniques such ascasting, blowing, and extrusion.

Good clarity in polyethylene blown film as indicated by low Haze andhigh Gloss has been noted in the past to be dependent upon severalfactors. Typically the Haze increases (and the Gloss decreases) as thepolymer density and molecular weight distribution increases. Also, ithas been noted that typically the surface roughness increases as themolecular weight distribution and density increases. Film stiffness onthe other hand, which is often a desired property of the blown filmdependent upon the actual application, has been noted to increase asdensity increases. Therefore, there has usually been a trade-off betweenfilm clarity and stiffness in polyethylene blown film.

Often in forming multi-layered films, a base layer of high molecularweight high density polyethylene or medium molecular weight high densityweight polyethylene has been employed to provide strength and a lowdensity polyethylene or linear low density polyethylene layer has beenprovided to provide other properties. Often, however, it has been notedthat the low density polyethylene and linear low density polyethylenelayers are tacky and sticky unless antiblock agents are included. Suchantiblock agents, however, generally also have an adverse effect uponthe clarity and physical properties.

An object of the present invention is to provide a method for producingfilms of ethylene polymers having a density of at least about 0.925 g/ccwhich have a good balance of processing, physical, and opticalproperties.

Other aspects, objects, and advantages of the present invention will beapparent from the following comments.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a unusuallyclear self-supporting film comprising at least one layer having apercent haze of less than 17.8 wherein the polymer of said layerconsists essentially of polyethylene having a density of at least about0.925 g/cc and a molecular weight distribution of no more than 4. Thenarrow molecular weight polyethylene having a density of at least about0.925 g/cc is preferably selected from polyethylenes which can be formedinto a 1 mil blown film having a percent haze of less than 17.8, or mostpreferably no more than 10.

In one preferred embodiment, the film has only one layer of polymerconsisting essentially of polyethylene having a density in the range of0.93 to about 0.945 g/cc and a molecular weight distribution in therange of about 1.5 to about 4, or more preferably about 1.5 to about3.5. In another preferred embodiment the film is multilayered and atleast one layer has a percent haze of less than 17.8, more preferably apercent haze of less than 10, and comprises polyethylene having adensity of at least 0.925 g/cc and a molecular weight distribution nomore than 4.

DETAILED DESCRIPTION OF THE INVENTION

The polyethylene useful for producing the inventive films can beproduced using a suitable metallocene-containing polymerization catalystsystem. In a particularly preferred embodiment the polyethylene isproduced in a slurry, i.e. particle form, type process wherein thepolymer is formed under conditions such that the polymer is produced inthe form of solid particles that can be readily separated from theliquid polymerization diluent. In such particle form polymerizations itis preferable that the metallocene-containing catalyst system beemployed in a form that is substantially insoluble in the polymerizationdiluent during the polymerization process. Various techniques are knownfor producing such relatively insoluble catalyst systems. Some examplesare shown in U.S. Pat. Nos. 5,354,721; 5,411,925; and 5,414,180.

One particularly preferred type of relatively insoluble solidmetallocene catalyst system can be produced by prepolymerizing a mixtureof a metallocene, preferably a metallocene having olefinicallyunsaturated substituents, and a suitable cocatalyst in the presence ofan olefin, generally containing 2 to 8 carbon atoms. In particularlypreferred embodiment the solid catalyst system is obtained bypolymerizing ethylene in the presence of an alkane liquid diluent underslurry polymerization conditions using a special type ofmetallocene-based catalyst system The catalyst system is a solidcatalyst prepared by (a) combining5-(9-fluorenyl)-5-(cyclopentadienyl)-hexene-1 zirconium dichloride andmethylaluminoxane in a liquid, (b) prepolymerizing ethylene in theresulting liquid, and (c) separating the resulting solid prepolymerizedcatalyst system from the liquid. It is preferred that the liquidemployed in step (a) be an organic liquid in which the methylaluminoxaneis at least partially soluble. Preferably some aromatic solvent isemployed in step (a). Examples of aromatic solvents include benzene,toluene, ethylbenzene, diethylbenzene, and the like. Preferably theamount of the liquid should be such as to dissolve the product ofreaction between the metallocene and the aluminoxane, provide desirablepolymerization viscosity for the polymerization, and to permit goodmixing. During the mixing, the temperature would preferably be keptbelow that which would cause the metallocene to decompose. Typically thetemperature would be in the range of about −50° C. to about 150° C.Preferably, the metallocene, the aluminoxane, and the liquid diluent arecombined at room temperature, i.e. around 10° C. to 30° C. The reactionbetween the aluminoxane and the metallocene is relatively rapid. Thereaction rate can vary over a wide range, however, it is generallydesired that they be contacted for an amount of time in the range ofabout 1 minute to about 1 hour.

It is also within the scope of the invention to carry out the step (a)in the presence of a particulate solid. Any number of particulate solidscan be employed. Typically this solid would be any inorganic solid thatdoes not interfere with the desired end results. Examples include poroussupports such as talc, inorganic oxides, resins to support material suchas particulate polyolefins. Examples of inorganic oxide materialsinclude metal oxides of Groups II-V, such as silica, alumina,silica-alumina, and mixtures thereof Other examples of inorganic oxidesare magnesia, titania, zirconia, and the like.

If a solid is employed, it is generally desirable for the solid to bethoroughly dehydrated prior to use. Preferably it is dehydrated so as tocontain less than 1 percent loss on ignition. Thermal dehydration may becarried out in a vacuum or while purging with a dry inert gas such asnitrogen at a temperature of about 20° C. to about 1000° C. andpreferably from about 300° C. to about 870° C. Pressure considerationsare not viewed as critical. The duration of the thermal treatment can befrom about 1 to about 24 hours as needed.

Dehydration can also be accomplished by subjecting the solid to achemical treatment in order to remove water and reduce the concentrationof surface hydroxyl groups. Chemical treatment is generally capable ofconverting all water hydroxyl groups in the oxide surface to relativelyinert species. Useful chemical agents are for example, carbon monoxide,carbonyl sulfide, trimethylaluminum, ethyl magnesium chloride, chlorosilanes such as SiCl₄, disilazane, trimethylchlorosilane, dimethylaminotrimethylsilane, and the like.

The amount of aluminoxane and metallocene used in forming a liquidcatalyst system for the prepolymerization can vary over a wide range.Typically, however, the molar ratio of the aluminum in the aluminoxaneto the transition metal of the metallocene is in the range of about 1:1to about 20,000:1, more preferably a molar ratio of about 50:1 to about2,000:1 is used. If a particulate solid, i.e. silica, is used, generallyit is used in an amount such that the weight ratio of the metallocene tothe particulate solid is in the range of about 0.00001/1 to 1/1, morepreferably 0.0005/1 to 0.2/1.

The prepolymerization is conducted in the liquid catalyst system, whichcan be a solution, a slurry, or gel in a liquid. A wide range of olefinscan be used for the polymerization. Typically, however, theprepolymerization will be conducted using an olefin, preferably selectedfrom ethylene and non-aromatic alpha olefins, such as propylene. It iswithin the scope of the invention to use a mixture of olefins, forexample, ethylene and a higher alpha olefin can be used for theprepolymerization. The use of a higher alpha olefin, such as 1-butene,with ethylene, is believed to increase the amount of copolymerizationoccurring between the olefin monomer and the olefinically unsaturatedportion of the metallocene.

The prepolymerization can be conducted under relatively mild conditions.Typically this would involve using low pressures of the olefin andrelatively low temperatures designed to prevent site decompositionresulting from high concentrations of localized heat. Theprepolymerization typically occurs at temperatures in the range of about−15° C. to about +150° C., more typically in the range of about 0° C. toabout +30° C. The amount of prepolymer can be varied but typically wouldbe in the range of from about 1 to about 95 weight percent of theresulting prepolymerized solid catalyst system, still more preferablyabout 5 to about 80 weight percent. It is generally desirable to carryout the prepolymerization to at least a point where substantially all ofthe metallocene is in the solid rather than in the liquid, since thatmaximizes the use of the metallocene.

After the prepolymerization, the resulting solid prepolymerized catalystis separated from the liquid reaction mixture. Various techniques knownin the art can be used for carrying out this step. For example, thematerial could be separated by filtration, decantation, or vacuumevaporation. It is currently preferred, however, not to rely upon vacuumevaporation since it is considered desirable to remove substantially allof the soluble components in the liquid reaction product of theprepolymerization from the resulting solid prepolymerized catalystbefore it is stored or used for subsequent polymerization. Afterseparating the solid from a liquid, the resulting solid is preferablywashed with a hydrocarbon and dried using a high vacuum to removesubstantially all the liquids or other volatile components that mightstill be associated with the solid. The vacuum drying is preferablycarried out under relatively mild conditions, i.e. temperatures below100° C. More typically the prepolymerized solid is dried by subjectionto a high vacuum at a temperature of about 30° C. until a substantiallyconstant weight is achieved. A preferred technique employs at least oneinitial wash with an aromatic hydrocarbon, such as toluene, followed bya wash with a paraffinic hydrocarbon, such as hexane, and then thevacuum drying.

It is also within the scope of the present invention to add aparticulate solid to the liquid catalyst system after it has been formedand then to carry out the prepolymerization in the presence of thatsolid. Another option is to add a particulate solid of the typeaforementioned after the prepolymerization or after the solidprepolymerized catalyst system has been separated from the liquid.

This resulting solid prepolymerized catalyst system is capable ofpreparing polymers of ethylene having a fairly wide range of densities.Typically, in preparing the lower density versions, the ethylene ispolymerized in combination with a smaller amount, generally less than 20mole percent, of at least one other alpha olefin, generally containingabout 3 to about 10 carbon atoms, examples of which include aliphatichydrocarbons such as butene-1, pentene-1, hexene-1, 4-methylpentene-1,octene-1, and the like. The solid prepolymerized catalyst system can beemployed using slurry polymerization conditions. Typically thepolymerization temperature would be selected so as to provide slurrypolymerization conditions in the particular liquid diluent selected.Typically the temperature would be in the range of about 20° C. to about130° C. With isobutane as the liquid diluent, temperatures in the rangeof about 60° C. to about 110° C. have been found desirable. Forproducing polymers for film applications, it is generally desirable toproduce a polymer having a melt index of less than 5. This can beaccomplished by adjusting the molar ratio of hydrogen to ethylene in thepolymerization process, changing the reactor temperature, and/orchanging the ethylene concentration.

When the polymerization is carried out in a continuous loop slurryprocess, it is generally desirable to include in the reaction mixture asmall amount of an antistatic agent. An example of such as antistaticagent is the material sold by DuPont Chemical Co. under the trade nameStadis 450.

In a particle form type polymerization the above described type ofcatalyst system is capable of producing polyethylene homopolymers andcopolymers having densities of 0.925 g/cm or higher with molecularweight distributions of no more than 4 that are useful for making filmshaving percent haze of less than 17.8, especially preferredpolyethylenes having densities in the range of 0.925 to 0.95 g/cc. Thepolymers produced in that manner have low flow activation energies, i.e.below about 25 kJ/mole, and a critical shear stress at the onset of meltfracture of less than 4×10⁶ dyne/cm². This is considered to indicatethat the polymers are substantially linear polymers substantially freeof long chain branching. The number of long chain branches in suchpolymers is considered to be less than 0.01/1000 carbon atoms. The term“long chain branching” as used herein refers to branches having a chainlength of at least 6 carbon atoms. A method of determining long chainbranching is disclosed in Randal, Rev. Macromol. Chem. Phys., C29 (243),285-297.

The ethylene polymers produced in a particle form process with thatcatalyst system are also believed to have a very uniform distribution ofshort chain branches both at the intramolecular level (monomer sequencedistributions along the chain) and at the intermolecular level (monomerdistribution between polymer chains of different molecular weights).Homopolymers and ethylene-hexene copolymers produced with such catalystsare particularly unusual in that they contain ethylene branches eventhough no butene comonomer was employed in the polymerization. It istheorized that butene is formed insitu in the polymerization and thatsuch results in a very uniform distribution of the ethylene branches.The shear stress response of such polymers is essentially independent ofthe molecular weight distribution.

It is typically desirable to add stabilizers to the polymer recoveredfrom the polymerization process. A number of suitable stabilizationpackages are known in the art. Stabilizers can be incorporated into thepolymer during a pelletization step or by reextrusion of previouslyproduced pellets. One example of a stabilizer would be Irganox® 1010antioxidant which is believed to be a hindered polyphenol stabilizercontaining tetrakis [methylene 3-(3,5-ditertbutyl4-hydroxy-phenylpropionate)] methane produced by Ciba-GeigyCorporation. Another example is the PEP-Q® additive which is a productof Sandoz Chemical, the primary ingredient of which is believed to betetrakis-(2,4-di-tertbutyl-phenyl)-4,4′ biphenyl phosphonite. Othercommon stabilizer additives include calcium stearate or zinc stearate.Still other stabilizers commonly used include Ultranox 626 antioxidantwhich is a product of GE, the primary ingredient of which is believed tobe bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite, and Ultranox627A antioxidant which is believed to be Ultranox 626 containing about 7weight percent of a magnesium aluminum hydrocarbonate. Such stabilizeradditives can be employed in generally any suitable amount The amountsused are generally the same as have been used for other polyethylenepolymers. Often the amounts for each additive is less than 0.2 weightpercent based upon the weight of the polymer.

The molecular weight of the polyethylene used to make the inventive filmcan vary over a wide range. Typically for forming films by blowing it isdesirable for the polymer to have a melt index in the range of about0.1-10 dg/min, more preferably about 0.2-5 dg/min. Generally if the meltindex of the polymer is less than about 1, it is often desirable toincorporate a processing enhancing amount of a fluoroelastomerprocessing aid. One example is the fluoroelastomer sold under the tradename Viton by E. I. DuPont de Nemours & Co. Another example is thefluoropolymer sold under the trade name Dynamar FX-9613 by 3M Company.The amount of fluoropolymer employed can vary over a wide rangedepending upon the particular results desired. Typically it would beemployed in an amount in the range of about 0.01 to about 1 weightpercent based upon the weight of the polyethylene. In some cases thefluoroelastomer is employed in form of a masterbatch in which thefluoroelastomer is dispersed in a polymer such as LLDPE copolymer ofbutene and ethylene. One example of such a material is Ampacet 10919processing aid masterbatch available from AMPACET Corp.

In some applications it may be desirable to include in the polymer ofone or more of the layers a slip/anti-block agent, particularly forlayers produced from polymers having a density of less than 0.925 g/cc.Generally such materials are inorganic compounds. Some examples includemica, talc, silica, calcium carbonate, and the like. A typical examplewould be Ampacet 10430 slip/antiblock concentrate available from AMPACETCorp.

It is also within the scope of the present invention for thepolyethylene used to produce the inventive films to contain variousother additives normally included in polyethylenes, such as heatstabilizers, weather stabilizers, lubricants, etc, in amounts that donot impact unduly on the objects of the present invention. It is alsowithin the scope of the present invention to blend the required narrowmolecular weight polyethylene having a density of at least about 0.925with other polymers so long as the amount of the other polymers does notunduly detract from the beneficial properties of the requiredpolyethylene, i.e. low haze and good handling properties. Generally therequired polyethylene is greater than about 50 weight percent of thepolymer, more typically at least 90 weight percent of the polymer, andstill more preferably at least about 99.5 weight percent of the polymer.

It is within the scope of the present invention to prepare single layerfilms having a haze of less than 17.8 using polyethylene having adensity of at least 0.925 and a molecular weight distribution of no morethan about 4. It is considered that such films can be produced bycasting, blowing, or extrusion.

It is also within the scope of the present invention to use such a layerof film to form a multilayered film. The polymers employed in the otherlayers can be selected from generally any of the polymeric materialsgenerally used in producing films. Thus the other layers need not belimited to polymers of ethylene but could contain other polymers such aspropylene-butene copolymer, poly(butene-1), styrene-acrylonitrile resin,acrylonitrile-butadiene-styrene resin, polypropylene, ethylene vinylacetate resin, polyvinylchloride resin, poly(4-methyl-1-pentene), andthe like. Multilayers can be formed using techniques generally known inthe art, such as, for example co-extrusion.

One particularly preferred example of a multilayered film includes onelayer having a percent haze of less than 17.8 comprising a polyethylenehaving a density in the range of about 0.925 to about 0.945 g/cc and amolecular weight distribution of no more than 4 and another layercomprising a second polyethylene having a molecular weight distributiongreater than 4, more preferably greater than 6, and still morepreferably greater than 10, such as polyethylenes produced usingPhillips chromium catalysts or Ziegler-Natta type catalysts.

For some applications it is also desirable for the polyethylene with thebroader molecular weight distribution to have a higher density than thepolyethylene having the narrower molecular weight distribution, forexample a density of at least about 0.945 g/cc. In a preferredembodiment of this type there are at least three layers and the outerlayers have a haze of less than 17.8 percent and comprise a polyethylenehaving a density in the range of about 0.925 to about 0.945 g/cc and amolecular weight distribution of no more than 4, and and the inner layercomprises a polyethylene having a density of at least about 0.945 g/cc.

In another preferred embodiment there are at least three layers and theouter layers have a haze of less than 17.8 percent and consistsessentially of polyethylene having a density in the range of about 0.925to about 0.945 g/cc and a molecular weight distribution of no more than4, and and the inner layer comprises polyethylene having a molecularweight distribution of at least 10 and a density of less than 0.93 g/cc,most preferably a density in the range of 0.91 to 0.929 g/cc with a HLMIin the range of about 12 to about 24 dg/min.

The most preferred multilayered films are those in which themultilayered film itself has a percent haze of less than 17.8, even morepreferably a percent haze of less than 10. In the currently preferredthree layer film the outer layers each have a thickness in the range ofabout 5 to about 25 percent of the total thickness of the three layeredfilm. A particularly preferred inner layer is one having a thicknessequal to about 50 to about 90 percent of the total thickness of thethree layered film, with the polymer of that inner layer being a lowdensity linear copolymer of ethylene and 1-hexene produced using aPhillips Cr catalyst in a particle form polymerization process,particularly a copolymer having a density in range of about 0.91 toabout 0.929 g/cc, an HLMI in the range of about 12 to 24 dg/min. and amolecular weight distribution greater than 10.

It is also within the scope of the present inventive mutilayered filmsto have a layer of polyethylene having a broader molecular weightdistribution and a lower density than the polyethylene in the layerhaving a percent haze of less than 17.8, for example one layer couldhave a percent haze of less than 17.8 and be composed of a polyethylenehaving a density at least 0.925 g/cc and molecular weight distributionof at least 4 and a second layer could be composed of a polyethylenehaving a density of less than 0.925 g/cc, such as for example a lowdensity polyethylene produced by a high pressure process.

It is also within the scope of the present invention to have amultilayered film in which one layer has a percent haze of less than17.8 wherein the polymer consists essentially of a polyethylene having adensity of at least 0.925 g/cc and a molecular weight distribution ofless than 4 and another layer composed of a low density polyethylenehaving a narrow molecular weight distribution and good clarity. In thatcase the inventive layer of polyethylene provides stiffness that may notbe provided by the lower density polyethylene without detracting fromthe clarity of the lower density polyethylene as much as would a similardensity polymer produced by a Phillips chromium catatalyst or aZiegler-Natta type titanium-containing coordination catalyst.

A layer having a percent haze of less 17.8 made of a polyethylene havinga density of less than 0.935 g/cc typically has a much lower meltingpoint than polymers of the same density and molecular weight produced byconventional transition metal coordination catalysts or Phillipschromium catalysts. If a lower melt temperature layer is desired it maytherefor be advantageous to use the polyethylenes having a density inthe range of 0.925 to 0.935 g/cc and a molecular weight distribution ofless than 4 to form the layer having the haze of less than 17.8.

In a particularly preferred embodiment all the polyethylene layers arepolyethylenes produced using metallocene catalysts which have molecularweight distributions of less than 4.

A further understanding of the present invention and its objects andadvantages will be provided by the following examples.

EXAMPLES Example I

A large batch of solid particulate metallocene-based catalyst wasprepared. The preparation involves reacting the metallocene (but-3-enyl)(cyclopentadienyl) (fluorenyl) (methyl) methane zirconium dichloridewhich is also known as (5-cyclopentadenyl) (5-fluorenyl) hex-1-enezirconium dichloride with a 10 weight percent solution ofmethylaluminoxane in toluene to give a soluble olefin polymerizationcatalyst system. Davison 948 silica, dried thermally and treated withtrimethylaluminum, was added to the liquid catalyst system. Toheterogenize this system the terminal unsaturated group of themetallocene was copolymerized with ethylene by adding ethylene tomaintain a pressure in the reaction vessel at 3 to 4 psig and stirringwhile the temperature was maintained at about 20° C. After about twohours, the ethylene addition was stopped and the slurry was filtered.The solid was washed with toluene and then with hexane and driedovernight using a membrane pump until no more solvent appeared on thecondenser. The resulting pink powder was dried an additional 5 hours ina high vacuum. The solid was sieved through a 60 mesh screen andcombined with Cabosil HS-5, a fumed silica which had been driedthermally and treated with trimethylaluminum.

The resulting solid metallocene-based catalyst system was then employedin a pilot plant scale continuous loop reactor under slurry typepolymerization conditions. The feedstocks to the reactor were passedthrough alumina drier beds prior to being sent to the reactor. Thereactor was a stainless steel pipe loop reactor. Circulation wasachieved by a propeller within the reactor. Reactant concentrations weremonitored by flash gas analysis using two on-line gas chromatographs.

The polymerizations were conducted in isobutane as a liquid diluentusing varying amounts of ethylene and hexene-1 comonomer to obtain anumber of lots of polyethylene fluff. Copolymers of ethylene andhexene-1 having densities varying from 0.9179 to 0.9402 g/cc wereproduced using the solid metallocene based catalyst system. Thepolyethylene copolymers of various densities were compounded with atypical stabilization package comprising 0.06 weight percent Irganox1010, 0.12 weight percent PEP-Q, and 0.05 weight percent zinc stearatebased upon the weight of the polymer.

The resulting polymers were then evaluated for various physicalproperties and were employed in the production of films using a 4 inchSano blown film line having a 1.5 inch single screw extruder. The filmdie is a spiral mandrel die with four entry ports and is 4 inches indiameter. The die had a dual lip air ring mounted on it which was usedto cool and stabilize the extruded bubble. Film blowing parameters wereemployed that are typical of linear-low density polyethylene typeprocessing conditions, including a 0.06 inch die gap, 190° C. extruderbarrel and film die set temperatures, 2.5:1 blowup ratio, no stalk, i.e.“in-pocket extrusion” in 1 mil film thickness. The screw rotation wasadjusted to keep the extrusion rate between 55 and 60 pounds per hour,so that the film properties so obtained would scale directly (i.e., bethe same as or at least very similar) with those obtained from larger,commercial scale equipment.

For some of the polyethylene copolymers runs were also made where thecopolymer had been compounded with 0.07 weight percent of FX-9613fluoropolymer. As controls films were also produced using thecommercially available Dow 2045A copolymer, which is believed to be alinear low density polyethylene copolymer produced using anon-metallocene titanium-based catalyst system. Also, films were madeusing a copolymer produced by a Phillips chrome resin.

Various characteristics of the polymer and the polymerization werecharacterized. Examples of characteristics determined in various casesinclude Haze (ASTM D-1003 using an XL-211 Hazeguard System fromGarder/Neotec Instruments Division); density in grams/mL (ASTM D1505-68); High Load Melt Index (HLMI) in grams of polymer/10 minutes190° C. (ASTM D1238-86, Condition 190/21.6); Melt Index (MI) in grams ofpolymer/10 minutes 190° C. (ASTM D1238-86, Condition 190/2.16); ShearStress Response (SR) determined by diving HLMI by MI; Molecular weightsby size exclusion chromatography, i.e. weight averge molecular weightreferred to herein as M_(w) and number average molecular weight referredto herein as M_(n); and Heterogenity index (HI) or molecular weightdistribution (MWD) being determined by dividing M_(w) by M_(n). The(SEC) size exclusion chromatography was conducted using a linear columncapable of resolving the wide range of molecular weights generallyobserved in polyolefins, such as polyethylene.

The property referred to herein as flow-activation energy, alsosometimes referred to as energy of activation, i.e. Ea, reflects thesensitivity of a polymer melt viscosity to temperature. This isgenerally viewed as a function of the linear vs network character of thepolymer. The molecular weight and the molecular weight distribution arealso generally viewed as factors affecting the flow activation energy.The Ea in terms of kJ/mol can be readily determined from data obtainedfrom a dynamic rheometer such as Rheometrics Inc. (RMS 800) dynamicrheometer. A standard prescription for summarizing theviscosity-temperature dependence of polymer melts has long beenavailable in the scheme known as the Williams-Landel-Ferry (WLF)superposition which is described in the classic text entitled“Viscoelastic Properties of Polymers”, 3rd Edition (John Wiley & Sons,New York, 1980) by John D. Ferry. Data needed for establishing thetemperature dependence of dynamic viscosity versus frequency, orviscosity vs shear rate, are not difficult to obtain at varioustemperatures in a range between melting and the onset of chemicaldegradation. In order to ensure that the Ea values are most accurate, itis desirable to optimize the data to produce optimally smooth isothermalmaster curves according to the WLF time-temperature superposition butusing a least squares closeness-of-fit criterion based on Carreau-Yasudamodel parameters that have been shown previously to give highly precisefits to single temperature polyethylene data. This can be done invarious ways. The currently preferred technique involves subjecting thedynamic viscosity frequency curves obtained from a Rheometrics, Inc.dynamic viscometer to a proprietary computer program entitled “RheologyAnalysis Program CY” covered by Phillips Petroleum Company unpublishedcopyright which was filed for registration on Jan. 31, 1995. Thisproprietary computer program is available for use by others under alicensing program.

Discussions of the Carreau-Yasuada model can be found in Dynamics ofPolymeric Liquids, Second ed. (John Wiley & Sons, New York, 1987) by R.Byron Bird, Robert C. Armstrong, and Ole Hassager; as well in C. A.Hieber and H. H. Chiang, “Some correlations involving the shearviscosity of polystryrene melts,” Rheol. Acta, 28, 321-332 (1989) and C.A. Hieber and H. H. Chiang, “Shear-rate-dependence modeling of polymermelt viscosity,” Polym. Eng. Sci. 32, 031-938 (1992).

The copolymers produced using the metallocene-based catalyst system havesome distinct differences from the Dow 2045A polymer and the polymerproduced using a Phillips chromium catalyst. Specifically, the polymersproduced using a metallocene-based catalyst had molecular weightdistributions in a range of 2.17 to 2.31 and unusually low meltingpoints for their density. The Dow polymer had a broader molecular weightdistribution. The polymer produced using a Phillips chromium catalyst amolecular weight distribution that was even broader than that of the Dowpolymer. In addition, the SR or HLMI/MI for the polymers produced usingthe metallocene-based catalyst were in the range of 17 to 18 whereas theDow resin was 30. From rheological data and Carreau-Yasuda parameters at190° C., the flow activation energies of the polymers were compared. Thepolymers produced from the metallocene-based system had flow activationenergies in the range of 20.48, to 23.71 kJ/mol. The Dow 2045A polymerin contrast had a flow activation energy, Ea, of 25.47 kJ/mol. Themetallocene-based polymers were also evaluated to determine theconcentration of terminal vinyl groups. The percent of chains with aterminal vinyl were in the range of 30 to about 42.9 percent, a value ofwhich is somewhat lower than that normally observed for copolymersproduced using chromium type catalysts. Carbon ¹³NMR analysis alsoindicated that the metallocene-based polymers showed the evidence oftrace amounts of ethyl and butyl short chain branches which may havecome from in-situ generated one olefin oligomers. As determined by FTIRspectroscopy, the total branching of the metallocene produced resinsvaried from about 0.4 to about 2.1 mole percent. The number of vinylgroups per 1000 carbon atoms for the metallocene based resins asdetermined by FTIR was in the range of 0.087 to 0.145.

A summary of the polyethylene properties and the properties of selectedfilms is shown in the following table.

Polyethylene Properties Film Properties Density Dart, MD TD Haze, Gloss,Film g/cc MI MWD g Tear, g Tear, g % % 1A 0.9179 1.06 2.17 388 200 3984.06 119.7 1B 0.9179 1.06 2.17 708 299 429 3.73 134.3 2A 0.9216 1.362.24 169 237 411 5.9 111.5 3A 0.9222 1.89 2.21 256 253 429 — — 3B 0.92221.89 2.21 145 174 453 5.66 118.2 4A 0.9256 0.98 2.31 153 170 422 — — 4B0.9256 0.98 2.31 152 222 355 — — 5A 0.9402 0.87 2.31  30  19 147 — — 5B0.9402 0.87 2.31 <30  24 168 5.74 121.4 Dow 0.9200 1.00 4.17 216 461 75517.8 — 2045 Cr 0.9230 — 24.0 — — — 27.08 30 Resin

In the above table if there is an A after the film number, it refers toa film prepared without any fluoroelastomer, whereas if there is a Bafter the number, it refers to a film produced using a polymercontaining 0.07 weight percent fluoroelastomer. No fluoroelastomer wasused in the control runs where films were produced from the Dow resinand the Phillips chromium resin.

The table demonstrates that in some cases the addition of fluoropolymerimproved the dart impact strength. It is important to note that themetallocene based resin was much clearer and smoother than the film ofthe resin with lower density that was produced with a Phillips chromiumcatalyst. While the metallocene resin having a density of 0.9402 g/cchad somewhat lower values for dart impact and tear resistance, the factstill remains that the copolymer produced using the metallocene iscapable of producing very clear films at densities much higher than thatnormally employed in making films. In addition films made from thehigher density resins have the additional property of greater stiffnessthan the films made from lower density polymer, a definite advantage insome applications.

It was further noticed that the films produced from the lower densitymetallocene based resins, i.e. those having a density of less than 0.925g/cc exhibited significant friction in the wooden take-up slats. Inaddition, the tackiness and blocking decreased as resin densityincreased. Accordingly, for the best balance of processing and clarityproperties, the metallocene produced resins having a density of at leastabout 0.925 g/cc were preferable. Additional runs were made thatdemonstrated that it was possible to produce 0.5 mil films using thespecial polyethylene copolymers having a density of at least about 0.925g/cc and a narrow molecular weight distribution.

Example II

A coextruded blown film having three layers was produced using a mediumdensity metallocene prepared using the same type of catalyst systemdescribed in Example I and a low density linear polyethylene producedusing a Phillips chromium catalyst process. Both ethylenes werecopolymers of ethylene and 1-hexene. The medium densitymetallocene-produced polymer had a density of 0.9309 g/cc and a meltindex of 0.87 dg/min. The low density linear polyethylene produced withthe Phillips chromium catalyst process had a density in the range of0.919 to 0.923 and a HLMI in the range of 15 to 21 dg/min. If oneproduced a 1 mil film using the chromium low density linearpolyethylene, it is possible to obtain good physical properties,however, the optical properties are less than would be desirable forclear film applications, i.e. the percent haze is greater than 17.8 insuch a film. A 1 mil film produced using the metallocene catalyst systemhad lower tear resistance than the low density linear polyethyleneproduced using the chromium catalyst. The 1.5 mil coextruded film wasextruded using a Sano coextrusion dye. Processing parameters included3.0:1 blow up ratio, 0.060 inch die gap at 200 lb/hour rate. The bubbleconfiguration was “pocket”. The process was carried out to produce aproduct in which 60 percent of the thickness was the low density linearpolyethylene and the two outer layers each were 20 percent of thethickness, the two outer layers being the metallocene polyethylene. Themetallocene polyethylene was compounded with 1 weight percent of Ampacet10919, which is believed to be a butene-ethylene linear low densitypolyethylene containing about 3 weight percent of the fluoroelastomerprocessing aid. A comparison of various properties of approximately 1mil films of each of the two resins and of the 1.58 mil coextruded filmare set forth in the following table.

Comparison of Films Metallocene Property Tested Coextruded Cr PolymerPolymer Gauge mil 1.58 1.01 1.08 E. Tear MD g 101 103 58 E. Tear TD g685 323 272 T.E.D.D. ft-lbs 1.23 1.45 0.886 Dart g 96 216 110 Ten. @Yield Md psi 1800 — 2150 Ten. @ Yield TD psi 1850 — 2300 Ten. @ Break MDpsi 4450 — 3750 Ten. @ Break TD psi 4350 — 4150 Elongation MD % 517 —506 Elongation TD % 723 — 630 Haze − 7.4 >17.8 4.4 Gloss − 115.6 — 129

The data shows that the coextruded film has improved optical propertiesas compared to the low density linear chromium based polyethylene andimproved properties in toughness as compared to the films made only fromthe metallocene polymer. Of particular note is the fact that the haze ofthe coextruded film is significantly lower than that of the polymer ofthe inner layer.

That which is claimed:
 1. A film containing three layers wherein thepolymer of each outer layer is a low haze layer having a percent haze ofless than 10% wherein the polymer consists essentially of polyethylene,optionally containing a fluoroelastomer, a density of at least 0.93g/cc, and a molecular weight distribution of no more than 4, saidpolyethylene being further characterized by being substantially free ofbranches having six or more carbon atoms.
 2. A film according to claim 1wherein the polyethylene of the inner layer has a density less than thatof the two outer layers.
 3. A film according to claim 2 wherein thepolyethylene of the inner layer has a molecular weight distribution ofat least
 10. 4. A film according to claim 3 wherein the polyethylene ofthe two outer layers is selected from polyethylenes having a density inthe range of about 0.94 to about 0.945 g/cc.
 5. A film according toclaim 4 wherein the polyethylene of the inner layer has a density in therange of about 0.91 to about 0.929 g/cc and an HLMI in the range ofabout 12 to 24 dg/min.
 6. A film according to claim 5 wherein thepolyethylene of the two outer layers is selected from the same ordifferent polyethylene selected from polyethylenes having a melt indexin the range of 0.2 to 5 dg/min.
 7. A film according to claim 6 which isa coextruded blown film having a percent haze of less than 10 percentand a thickness in the range of 0.25 to 5 mil.
 8. A film according toclaim 7 wherein at least one of the outer layers contains polyethylenehaving a melt index of less than about 2 dg/min and said polyethylenecontains about 0.01 to about 1 weight percent fluoroelastomer.
 9. Acoextruded blown film having a percent haze of less than 17.8%consisting of three layers wherein the polymer of the each outer layersis the same or different polyethylene selected from polyethylenes havinga density of at least 0.925 g/cc, and a molecular weight distribution ofless than 4 and being further characterized by being substantially freeof branches having six or more carbon atoms, and the polymer of theinner layer consists essentially of a copolymer of ethylene and 1-hexenehaving a density in range of about 0.91 to about 0.929 g/cc, an HLMI inthe range of about 12 to 24 dg/min. and a molecular weight distributiongreater than 10 produced using a chromium oxide containing catalyst in aparticle form polymerization process.
 10. A film according to claim 9having a haze of no more than 10% wherein the polymer of the outerlayers is selected from copolymers of ethylene and 1-hexene having adensity of at least about 0.93 g/cc, wherein the thickness of each outerlayer is in the range of about 5 to about 25 percent of the totalthickness of said film, and wherein if the polymer of each of the outerlayers was formed into a one mil film the films would each have a lowerhaze than a one mil film produced from the polymer of the inner layerunder the same conditions.